The present invention relates to an image reading apparatus suitable for use in facsimile terminal equipment, and more particularly to an improvement in picture quality of image information obtained by scanning an original matter.
Conventional facsimile terminal equipment is provided with two kinds of binarization units for carrying out binarization and pseudo gray scale processing, and a user selects one of two kinds of processing in accordance with an original to be read (refer to the instruction manual of Hitachi high-speed facsimile HIFAX 130M). The reason for this is as follows. In a case where the binarization processing is carried out, the tone of a halftone region on a photograph or the like cannot be expressed. On the other hand, in a case where the pseudo gray scale processing is carried out, it is impossible to reproduce a character or line in a satisfactory manner. Further, as to processing for improving the picture quality of image information, in order to prevent a character, a line, or the like from being destroyed or becoming light, contour enhancement processing is carried out for image information obtained by scanning an original. When the contour enhancement processing is carried out for a dot region, however, a Moire pattern is often generated in a binary output. A method of preventing the generation of a Moire pattern is proposed in a Japanese Patent Application Un-examined Publication JP-A-2-292,956. In this method, a dot region is detected to carry out smoothing processing for the dot region, and contour enhancement processing is carried out for regions other than the dot region. Thus, the generation of a Moire pattern in the dot region is prevented, and the sharpness of characters is improved in a character region. Further, another method of preventing the generation of a Moire pattern is proposed in a Japanese Patent Application Un-examined Publication JP-A-2-168,771. In this method, the smoothing processing is first carried out for image information, and then the contour enhancement processing is carried out. Thus, the generation of a Moire pattern in a dot region is prevented, and the sharpness of a character is improved.
As mentioned above, in the conventional facsimile terminal equipment, one of two kinds of binarization processing is selected in accordance with the original used. Now, consider a case where an original, on which a dot region and a character region coexist, such as a newspaper is used. When the binarization processing is selected, the halftone of the dot region cannot be expressed, and moreover a Moire pattern is generated in the dot region by the contour enhancement processing. On the other hand, when the pseudo gray scale processing is selected, each of a character and a thin line is output in a discontinuous form. In either binarization processing, a large number of black or white discrete pixels are generated which do not exist in the original. Thus, when a coding method for reducing the amount of information by paying attention to the continuity of pixels, for example, Modified Huffmann (MH) coding method or Modified Relative Element Address Designate (MR) coding method is used, there arise problems that the amount of code is increased and a transmission time is also increased.
Further, when the method of preventing the generation of a Moire pattern by detecting a dot region is used to improve the picture quality of an output image, there arises a problem that an unclear image region such as a dot region including character information is misjudged, and thus an output image is deteriorated. In order to solve this problem, a large-scale, dot-region discriminating circuit is required. When the method of preventing the generation of a Moire pattern is carried out, there arise the following problems. In the smoothing processing, the density values of pixels adjacent to a target pixel in horizontal, vertical and oblique directions are averaged. Accordingly, even when the contour enhancement processing is carried out for a thin line whose width is nearly equal to the width of one scanning line, the thin line cannot be restored. Further, in a case where a region including n pixels in a traverse direction is required for smoothing processing and a region including m pixels in a traverse direction is required for contour enhancement processing, a memory is necessary which can store image data corresponding to at least (m+n-1) lines.